Athens is about 400 km or so from Rhodes (as the crow flies).
Are you planning to drive? If so the journey will take just over 21 hours. This website will give you the driving directions as well as a map.
http://local.live.com/default.aspx?wip=2&v=2&style=r&rtp=~&&msnurl=home.aspx?%26redirect%3dfalse&msnculture=en-US
The other way is to take a ferry. Here is a map showing you the ferry services. There are daily passenger and vehicle ferries - about 18 hours.
http://www.aegeantourtravel.com/ferries/map-aegeanseaferries.html
Athens to Rhodes by air is only 55 min:
http://milos.ipta.demokritos.gr/Harmo/how_to_reach_rhodes.htm
Edited by GemB on August 6 2006 at 12:42 PM
Hope this helps and that you will accept my help!